1. Field of the Invention
This invention relates to a position detecting device using magnetism.
2. Description of the Prior Art
It has been a common shortcoming of conventional magnetic position detecting devices that, where a code on a magnetic code plate, such as a magnetic recording tape or the like is long, the output level of the device drops at points of the code other than the beginning and end. Where a magnetic code plate 2 is provided with Gray codes as shown in FIG. 1 of the accompanying drawing, and, for example the device is an absolute position detecting device, it has been impossible to read higher place bits of some codes when the code is a long signal (tracks B and C in FIG. 1). More specifically, when the length of a magnetized pattern (or code) is long, as represented by the track B in FIG. 1, for example, the output of the device obtained as a result of reading with a magnetic sensor 1 is shown in FIG. 2. The output for the address 13 of the track B, for example, becomes "0", which is the same as the output obtained when the sensor 1 is at another address 2 shown in FIG. 1. Such an error causes confusion. Further, the sensor 1 is arranged to produce "1" at a magnetized part and "0" at a part which is not magnetized (the parts not hatched in the drawing). In FIG. 1, the magnetized parts are of N polarity. However, magnetization may be of S polarity. In that event, the sensor produces "-1" instead of "1". In other words, the conventional magnetic position detecting devices have required some additional arrangement to prevent the adverse effect of self-demagnetization.